Exhaust port tap

toppy · Dec 2, 2014

tricatcent said:
A really good way to clean up big internal threads when they have been cross threaded or if they are just dirty or dinged up, is to get a tap with the same number of threads per inch as the thread you want to clean up. I can't remember how many threads per inch the Norton exhaust nuts are but for example if they are 14 threads per inch, you would use a 7/16-14 UNC tap. It would be best if you can find a plugging tap. Then you put a handle on it and use it as a scraper. I have done this a few times and found it works amazingly well. Usually I seem to have a tap already in my tool box, but even if you have to buy a small tap like this it is not expensive.

Nigel

I believe that Nigel means if the tap has the same thread form and pitch that you can use it like a hand chaser that I suggested. I to do have also done this many times with great success when a hand chaser or tap was not available.

daveparry · Dec 2, 2014

Yes I now realize what Nigel meant.

kommando · Dec 2, 2014

I have just used a 1/4 UNF 28tpi tap to clean up the threads on some 1970 Triumph Shuttle valve stanchions, they were missing the undercut and when I used the lathe the first thread was messed up, the tap restored the thread, you just need to make sure you engage lots of threads so they lead the tap not the duff thread.

tricatcent · Dec 2, 2014

Another trick that is worth knowing to do with thread files and also internal thread scraping using an internal thread file or a tap, is that if you have a file or tap with half the thread pitch you need, you can also use that. A lot of British machines have CEI threads that are 26 TPI. You can use a 1/2NC 13 tap as an internal scraper or a 13 TPI file to clean up an external thread. You just have to make sure you move the scraper so that you get all the threads.

triumph2 · Dec 2, 2014

On the subject of retapping threads if the part is aluminum then the use of a forming tap would be advisable as they don't remove any material. In cast aluminum, forming a new thread will actually be stronger than using a cutting tap.

RennieK · Dec 2, 2014

I've always figured at one time these threads were whitworth so - 14 tpi @55'. If so does anyone know when Norton changed over to UN, 14 tpi @ 60'?

and I thought 1.997 tpi was closest to 1 59/64 tap size or do you just chase the threads when the engine is hot with the 1 15/16 tap?

Exhaust port tap

toppy

daveparry

kommando

tricatcent

triumph2

RennieK

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